Rudolf Diesel developed the first diesel engine and obtained a German patent for it in 1892. His goal was to build an engine with high efficiency. Gasoline engines had been invented in 1876 and, especially at that time, were not very efficient.
Unlike gasoline engines that ignite mixtures of gas and air with a spark, a diesel engine intakes air, compresses the air, and injects fuel into the compressed air, such that the heat and pressure of the compressed air ignites the fuel spontaneously. Diesel engines do not have spark plugs or other ignition sources. Some older diesel engines include glow plugs to warm the cylinders in cold conditions, but the glow plugs are not ignition sources; rather, they are resistive warming elements.
Pistons of typical gasoline engines compress at a ratio of between 8:1 and 12:1, while a diesel engine normally compresses at a ratio of 14:1 to 25:1. The higher compression ratio of the diesel engine leads to more torque and better fuel efficiency. The use of diesel fuel allows the compression ratios of diesel engines to be much higher than for gasoline engines. Gasoline auto-ignites at lower temperatures and pressures that diesel fuel, and auto-ignition results in knock in gasoline engines.
Diesel fuel has a higher auto-ignition temperature than gasoline and is heavier and oilier than gasoline. Diesel fuel evaporates much more slowly than gasoline—its boiling point is actually higher than the boiling point of water. Diesel fuel contains more carbon atoms in longer chains than gasoline does (gasoline is typically primarily C9H20, while diesel fuel is typically primarily C14H30). Crude oil also requires less refining to create diesel fuel, which is why diesel fuel is generally cheaper than gasoline.
Diesel fuel also has a higher energy density than gasoline. On average, one gallon (3.8 L) of diesel fuel contains approximately 155×106 joules (147,000 BTU) of energy, while one gallon of gasoline contains 132×106 joules (125,000 BTU) of energy. This higher energy density, combined with the improved efficiency of high compression diesel engines, explains why diesel engines get better fuel economy than equivalent gasoline engines.
The fuel injector of a diesel engine is usually its most complex component and has been the subject of a great deal of experimentation—in any particular engine it may be located in a variety of places. The injector must withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Circulating the mist of fuel in the cylinder so that it is evenly distributed is also a common problem.
Even distribution of the diesel fuel within the cylinder and mixing the fuel with air contribute to the completeness of combustion of the diesel fuel. To optimize fuel oxidation within an engine's combustion chamber, the fuel/air mixture is ideally vaporized or homogenized to achieve a chemically-stoichiometric gas-phase mixture. Ideal fuel oxidation results in more complete combustion and lower pollution.
Relative to internal combustion engines, stoichiometricity is a condition where the amount of oxygen required to completely burn a given amount of fuel is supplied in a homogeneous mixture resulting in optimally correct combustion with no residues remaining from incomplete or inefficient oxidation. Ideally, the fuel should be completely vaporized, intermixed with air, and homogenized prior to entering the combustion chamber for proper oxidation. Non-vaporized fuel droplets generally do not ignite and combust completely in conventional diesel engines, which presents problems relating to fuel efficiency and pollution.
Incomplete or inefficient oxidation of diesel fuel causes exhaustion of residues from the diesel engine as pollutants, such as unburned hydrocarbons, carbon monoxide, and aldehydes, with accompanying production of oxides of nitrogen. To meet emission standards, these residues must be dealt with, typically requiring further treatment in a catalytic converter or a scrubber. Such treatment of these residues results in additional fuel costs to operate the catalytic converter or scrubber. Accordingly, any reduction in residues resulting from incomplete combustion would be economically and environmentally beneficial.
Aside from the problems discussed above, a fuel-air mixture that is not completely vaporized and chemically stoichiometric causes the combustion engine to perform at less than peak efficiency. A smaller portion of the fuel's chemical energy is converted to mechanical energy when fuel is not completely combusted. Fuel energy is wasted and unnecessary pollution is created. Thus, by further breaking down and more completely vaporizing the fuel-air mixture, higher compression ratios and better fuel efficiency may be available.
Many attempts have been made to alleviate the above-described problems with respect to fuel vaporization and incomplete fuel combustion. Diesel fuel injectors spray a somewhat fine fuel mist directly into the cylinder of the engine and are controlled electronically. Nevertheless, the fuel droplet size of a fuel injector spray is not optimal and there is little time for the fuel to mix with air prior to ignition. Even current fuel injector systems do not fully mix the fuel with the necessary air.
Moreover, it has been recently discovered that fuel injector sprays are accompanied by a shockwave in the fuel spray. The shockwave may prevent the fuel from fully mixing with air. The shockwave appears to limit fuel mass to certain areas of the piston, limiting the fuel droplets' access to air.